Melatonin, a neuroendocrine transducer of photoperiod, is used by mammals to establish and maintain circadian rhythms. Melatonin receptors which mediate these functions are believed to be located in the hypothalamus. Recent studies demonstrate that high affinity melatonin receptors are also present in cerebral arteries and that melatonin is capable of causing constriction of these arteries. However, our understanding of the functional role of melatonin receptors in cerebral arteries is lacking. Therefore, the major goal of the proposed research is to define the physiological role of these receptors in cerebral blood flow. The proposed project will focus on functional studies and signal transduction mechanisms of melatonin receptors in the cerebral arteries of rats, and pharmacological characterization of melatonin binding sites that the applicant has recently identified in human cerebral arteries. The specific aims of this proposal are: (1) Evaluation of the physiological role of melatonin in cerebral circulation. We will test the hypothesis that melatonin modulates cerebral vasodilation during experimental hypotension and hypercapnia, conditions where vasodilators are known to be active. (2) Evaluation of the pharmacological properties and receptor signaling mediated by melatonin receptors in cerebral arteries. We will test the hypothesis that: (a) melatonin causes vasoconstriction by potentiating the stimulating action of calcium-mobilizing agents on phosphoinositide phospholipid hydrolysis; (b) melatonin inhibits cAMP accumulation stimulated by vasodilators; and (c) a changes in density and affinity of melatonin receptors in the artery will be reflected in a significant change in melatonin-induced contraction. (3) Localization and characterization of melatonin receptors in human cerebral arteries. Using quantitative autoradiography we will perform the pharmacological characterization of melatonin receptors in various human cerebral arteries. These studies will (a) further our understanding of the role of melatonin receptors in cerebral arterial contraction and cerebral blood flow, and the biochemical processes by which these effects are produced, and (b) pave the way for future studies on the physiology of vascular melatonin receptors in health and disease.